Generally, washing machines are apparatuses which perform washing, rinsing, and spin-drying operations to do the laundry.
Washing machines, depending on types, are classified into pulsator type washing machines in which water currents generated when small blades at a bottom of a washing tub rotate give the laundry a shock to wash, agitator type washing machines in which a large impeller with vanes attached to a center of a washing tub regularly turns around while reversed in direction and forms water currents to wash, and drum type washing machines in which the laundry is inserted into a drum and the drum rotates, thereby doing the laundry using a shock caused by a head and detergency of a detergent.
Washing machines each include a cabinet, a tub installed inside the cabinet to store washing water, a washing tub rotatably installed in the tub to accommodate the laundry, a driving device which rotates the washing tub, a water supply device which supplies the washing water to the tub, and a drainage device which discharges the washing water from the washing tub to the outside of the cabinet when washing is completed.
Meanwhile, while washing is performed, a vibration and noise occur due to the washing tub rotated by the driving device. To reduce such a vibration and noise, a damper is installed.
Generally, in the case of dampers, a frictional member is fixed to a bar so that only one damping force may act. Accordingly, during a spin-drying operation, vibration transfer to the outside increases in a transient vibration section of a low speed spin zone in which vibration displacement is great and driving is performed for a short time and a normal vibration section of a high speed spin zone in which vibration displacement is small and driving is performed for a long time, thereby generating a vibration and noise of an exterior of a washing machine.